Plumes and Jets: Constraints on Vents and Eruption Dynamics from Observations and Models

Abstract:

Plume activity of Enceladus has been monitored by Cassini for nearly one decade after their discovery (see Science, 2006, 311, special issue). Thus, crucial properties of the vapor dust plumes are constrained in a fairly detailed manner. In this paper I discuss implications for vent geometries, gas and grain dynamics and condensation in the vents.

Vapor source rates on the order of 100 to 1000kg/s were derived from remote and in-situ data [2, 3, 4, 1, 17] and distortions in the B field [10, 15]). Gas ejection speeds from 500m/s to 1000m/s [18, 4] (escape speed 240m/s) indicate supersonic gas flow. Evidence for supersonic gas jets is directly seen in UVIS data [3].

Dust production rates between 5 to 50kg/s have been inferred [16, 8]. These do not yet include mass in jets of very fine nano-grains [9, 8, 7]. The dust plume exhibits scale heights that suggest ejection speeds on the order of 100m/s [13, 16, 6], i.e. well below the escape velocity. Larger grains have smaller ejection spees populating the lower parts of the plume [6, 16]. Salt has been identified in grains on the percent level [14] so that they cannot form alone by condensation from vapor. The detailed distribution of dust sources and jet orientations on the south polar terrain was derived from images and compared to temperature distributions and to the expected tidal stress pattern from modelling [12].

A recent observations show that plume brightness varies roughly by a factor of three with the orbital period of Enceladus, suggesting that ejection strength is tidally controlled [11, 5]. A similar variation in the gas discharge is expected but has not yet been observed to date. Remarkably, there is no such correlation of orbital phase and the observed scale height of dust jets.

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